Magnetic elements

ABSTRACT

There are disclosed magnetic elements having unique shapes. In one example, the magnetic element defines an outer peripheral profile and a center point, wherein the outer peripheral profile includes a substantially curviform section and a notch section. The notch section may be configured to radially extend to at least the center point. In another example, a substantially circular or oval-shaped magnetic element defines an outer periphery and a gap void having an open end facing the outer periphery so as to form a gap along the outer periphery. The magnetic element optionally may not include an annular void that is spatially isolated from the gap void.

PRIORITY

This application claims priority to U.S. Provisional Application60/416,213, filed Oct. 3, 2002, which is incorporated herein byreference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with United States Government support underContract N00014-02-1-0991 awarded by the United States Department ofNaval Research. The United States Government has certain rights in theinvention.

FIELD

The present disclosure relates to magnetic elements, particularly thoseuseful for forming a magnetic tunneling junction.

BACKGROUND

A magnetic tunneling junction (MTJ) is formed by interposing a thininsulating layer between a pair of magnetic layers. When a voltage isapplied between the two magnetic layers, electrons in one of themagnetic layers enter the other layer by passing through the insulatinglayer by quantum mechanical tunneling. The electrical resistance of theMTJ varies according to the direction of magnetization of the twomagnetic layers. In particular, the electrical resistance of the MTJ hasa minimum value when the directions of magnetization of the two magneticlayers are in parallel with each other, and has a maximum value when thedirections of magnetization of the two magnetic layers are in ananti-parallel relationship. If the direction of magnetization of one ofthe magnetic layers is changed by an applied magnetic field (externalmagnetic field) while the direction of magnetization of the othermagnetic layer remains fixed, the electrical resistance value of the MTJchanges according to the direction of the applied magnetic field. Thischanging of the direction of the magnetization is referred to as“switching.” Information can be stored in each MTJ and read out (i.e.,reproduced) from the MTJ by sensing the tunneling current value.

One useful application of a MTJ is in a type of nonvolatile memorydevice known as a magnetic random access memory (MRAM) device. Eachmemory cell of the MRAM device incorporates a MTJ, and a plurality ofmemory cells are arranged in an addressable array. Magnetic tunnelingjunctions that have been employed in MRAM devices utilize magneticelements having solid elliptical or modified rectangular shapes (e.g.,hexagon). Such shapes exhibit a linear magnetization mode that producesseveral problems when they are scaled down to nanometer-sized elements.For example, edge magnetic domain, 360° domain walls, and localizedvortices will occur leading to multiple magnetic domains. Such multiplemagnetic domains cause unrepeatable switching and a non-uniformmagnetization configuration of the magnetic element. Another significantissue with conventional magnetic element geometries, especiallygenerally rectangular shapes with modified end portions, is theexistence of a wide switching field distribution. Consequently, a largeswitching error occurs on individual memory cells.

In order to address the deleterious effects of the above-describedmagnet element shapes, a circular or an asymmetric circular magnetelement has been proposed. However, for solid disc-magnetic elementshaving an asymmetric circular shape the vortex core interrupts thestability of the magnetic configuration in sub-micron-sized elements.More specifically, the vortex core is positioned on the center of asymmetrical disc, but in an asymmetrical design the location of thevortex core is not well defined. Consequently, the vortex core isunstable resulting in a high switching field and a wide switching fielddistribution. A ring-shaped magnetic element has also been proposed (seePrinz, U.S. Pat. No. 5,969,978). A ring-shaped magnetic element includesan isolated cavity or void that is difficult to accurately pattern withexisting fabrication processes.

SUMMARY OF THE DISCLOSURE

Disclosed herein are unique magnetic elements, and magnetic tunnelingjunctions constructed from such magnetic elements.

For example, there is disclosed a magnetic element defining an outerperipheral profile and a center point, wherein the outer peripheralprofile includes a substantially curviform section and a notch section.The notch section may be configured to radially extend to at least thecenter point. According to another variant, a magnetic element definesan outer periphery and a gap void having an open end facing the outerperiphery so as to form a gap along the outer periphery, wherein themagnetic element does not include an annular void that is spatiallyisolated from the gap void.

There is also described a first example of a magnetic tunneling junctioncomprising a first magnetic layer and a second magnetic layer. Each oneof the magnetic layers defines an outer peripheral profile and a centerpoint, wherein the outer peripheral profile includes a substantiallycurviform section and a notch section. In one variation the notchsection is configured to radially extend to at least the center point.The magnetic tunneling junction also includes an insulating layerinterposed between the first magnetic layer and the second magneticlayer.

A second example of a magnetic tunnel junction includes a substantiallycircular or oval-shaped first magnetic layer and a substantiallycircular or oval-shaped second magnetic layer. Each of the magneticlayers defines an outer periphery and a gap void having an open endfacing the outer periphery so as to form a gap along the outerperiphery. In one variation the magnetic layer does not include anannular void that is spatially isolated from the gap void. The magnetictunneling junction also includes an insulating layer interposed betweenthe first magnetic layer and the second magnetic layer.

The disclosed structures and methods will become more apparent from thefollowing detailed description of several embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments are described below with reference to the followingfigures:

FIG. 1 is a perspective view of a first disclosed embodiment of amagnetic element;

FIG. 2 is a perspective view of a second disclosed embodiment of amagnetic element;

FIG. 3 is a cross-sectional view of a third disclosed embodiment of amagnetic element;

FIG. 4 is a cross-sectional view of a fourth disclosed embodiment of amagnetic element;

FIG. 5 is a cross-sectional view of a fifth disclosed embodiment of amagnetic element;

FIG. 6 is a cross-sectional view of a sixth disclosed embodiment of amagnetic element;

FIG. 7 is a cross-sectional view of a seventh disclosed embodiment of amagnetic element;

FIG. 8 is a cross-sectional view of an eighth disclosed embodiment of amagnetic element;

FIG. 9 is a cross-sectional view of a ninth disclosed embodiment of amagnetic element;

FIGS. 10A, 10B, 10C, and 10D are atomic force microscopy (AFM)/magneticforce microscopy (MFM) images showing magnetization configurations ofseveral as-patterned magnetic elements;

FIGS. 11A-11H are MFM images of magnetization configurations of severalmagnetic elements at remanent state after applying successive magneticfields;

FIG. 12 is a schematic of a first wiring configuration for a novel MRAMdevice;

FIG. 13A is a schematic of a second wiring configuration for a novelMRAM device;

FIG. 13B is an enlarged schematic view of a MTJ wired according to theconfiguration shown in FIG. 13A;

FIG. 14 is a graph illustrating a remanent curve of the percentage ofunswitched magnetic elements vs. the applied magnetic field; and

FIGS. 15A-15C are cross-sectional views of further disclosed embodimentsof a magnetic element.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

The magnetic elements illustrated herein are generally curviform inoverall shape. As used herein, “curviform” means a shape or line thatinclude at least one curve. The circular or oval elements each include agap or notch portion that forms a void at their outer periphery.Consequently, the profile of the outer periphery of the elements definesa discontinuous curve. The void typically extends to at least the centerpoint of the curviform geometry of the elements. In other words, theremay be a single void that extends in a radial direction from at or nearthe center point to the outer periphery of the element. The magneticelements usually do not include a separate, annular void that isspatially isolated from the gap or notch void. A “spatially isolated”annular void means that there is magnetic structure between the annularvoid and the gap or notch void (i.e., the annular void and the gap ornotch void are not in spatial communication with other). Severalexamples of possible geometries are shown in FIGS. 1-9 and 15A-15C. Themagnetic elements may have other geometries in addition to thoseexemplified in the drawings.

With reference to FIG. 1, there is shown a circular magnetic element 1that defines an outer periphery 2 that includes a curviform section 3and a notch or gap section 4. The notch section 4 in FIG. 1 is generallywedge-shaped. The wedge-shaped notch section 4 defines an open end 5that tapers into a closed end 6. The angle α formed by the wedge mayrange, for example, from about 1 to about 180° (see FIG. 15C which showsan element in which the angle α is 180°). The closed end 6 may belocated at or near a center point CP of the circle formed by the outerperiphery 2 of the element 1.

With reference to FIG. 2, there is shown another circular magneticelement 1 in which the notch section 4 defines a slot profile having anopen end 11 facing the outer periphery 2 and a closed arcuate end 12located at or near a center point CP of the circle formed by the outerperiphery 2 of the element 1.

FIGS. 3-9 show other geometry variations for the magnetic element 1. Themagnetic element 1 in FIG. 3 includes a wedge-shaped notch section 4that defines an arcuate closed end 12. The magnetic element in FIG. 4assumes a half-moon geometry in which the notch section 4 defines aparabolic-shaped profile 13. The center point CP of the half-moon isalso identified in FIG. 4. The magnetic element in FIG. 5 has agenerally oval or elliptical shape rather than a circular shape, andincludes a notch section 4 similar to that shown in FIG. 2. The magneticelement in FIG. 6 also has a generally oval shape but the notch section4 is aligned parallel to the radial direction of the longer diameter ofthe oval shape. The magnetic element in FIG. 7 includes a notch section4 that defines a slot portion 14 that communicates with a circularportion 15. The circular portion 15 may be concentric with the centerpoint of the element or it may be off-center from the center point. Themagnetic element in FIG. 8 has a generally oval shape rather than acircular shape, and includes a notch section 4 similar to that shown inFIG. 7. The magnetic element in FIG. 9 also has a generally oval shapebut the notch section 4 is aligned parallel to the radial direction ofthe longer diameter of the oval shape. The center point of the ovalshapes in FIGS. 8 and 9 is designated “CP.” In additional variationsshown in FIGS. 15A-15C, the outer periphery 2 of the magnetic element 1further includes a straight section 7 opposing the notch section 4.

The magnetic elements may have any dimensions that are required to meetthe desired performance properties. For example, the outer radius R (seeFIG. 2) from the center point of the curviform element geometry to theouter periphery may range from about 50 to about 1000 nm, moreparticularly about 100 to about 900 nm. The inner radius r (see FIG. 2)from the center point of the curviform element geometry to the arcuateor circular closed end of the notch section 4 may range from about 20 toabout 900 nm, more particularly about 100 to about 800 nm. The thicknesst (see FIG. 2) of the element may range from about 5 to about 500 nm,more particularly about 10 to about 100 nm. The gap distance d (i.e.,the width of the slot section (see FIG. 2)) may range from about 20 toabout 900 nm, more particularly about 50 to about 800 nm. The distance afrom the center point of the curviform element geometry to the straightsection 7 (see FIGS. 15A-15B) may be about one-third of the outer radiusR. With respect to oval-shaped magnetic elements, the width or longdimension may be about 75 to about 1500 nm and the height or shortdimension may be about 25 to about 500 nm.

The magnetic elements may be made from any magnetic-responsive materialsuch as a ferromagnetic or ferrimagnetic material. Illustrativematerials include cobalt-containing materials, nickel-containingmaterials, ferrous materials, chromium-containing materials, and alloysor mixtures made therefrom. Specific materials include Co, Co₅₀Fe₅₀,CrO₂, Fe₃O₄, Ni, NiFe (e.g., Ni₈₀Fe₂₀), Ni₅₀Fe₃₀Co₂₀, Fe, Co CrO₂,Fe₃O₄, and La_(0.67)Sr_(0.33)MnO₃.

The magnetic elements are particularly useful in nonvolatile MRAMdevices that include a MTJ. As described above, a MTJ includes a firstmagnetic layer and a second magnetic layer. Both the first magneticlayer and the second magnetic layer are provided with a geometricalconfiguration in accordance with the magnetic elements described herein.An insulating layer is interposed between the first magnetic layer andthe second magnetic layer. The geometrical configuration of theinsulating layer may be the same as, or different from, the geometricalshape of the first and second magnetic layers.

The insulating layer generally has a thickness ranging from about 1 toabout 10 nm. The insulating layer may be made from any magneticallyinsulating or non-magnetic material such as, for example, an aluminumoxide (e.g., Al₂O₃), nickel oxide, tantalum oxide, magnesium oxide,hafnium oxide, gadolinium oxide, aluminum nitride, aluminum oxynitride,boron nitride, zirconium oxide, BaSr_(1-x)Ti_(x) (x=0.00 to 1), titaniumoxide, and mixtures thereof. The composition of the first magnetic layermay be the same as or different than the composition of the secondmagnetic layer. The thickness of each of the first magnetic layer andthe second magnetic layer may generally be the same as described abovefor the thickness t of the magnetic element.

One of the magnetic layers in the MTJ is known as the “free layer” sinceit is capable of reversing magnetization upon the direction of anapplied, relatively small, magnetic field. The other magnetic layer inthe MTJ is known as the “pinned layer” since its direction ofmagnetization remains constant while the free layer is undergoing achange in magnetization direction. Typically, the free layer iscomprised of a “soft” magnetic material that can undergo magnetizationdirection reversal under a relatively lower magnetic coercive force. Thepinned layer is comprised of a “hard” magnetic material that requires arelatively higher magnetic coercive force to induce a change in themagnetization direction.

The MTJ may be made by any technique used in forming nanometer layers inmicroelectronic devices. For example, the first magnetic layer isprovided on a suitable substrate (e.g., glass, silicon, germanium,gallium arsenide, or mica) by a deposition technique such as chemicalvapor deposition (e.g., plasma-enhanced chemical vapor deposition orvapor phase epitaxy) or physical deposition (e.g. sputtering,electroplating or evaporation). The insulating layer is provided on thefirst magnetic layer by another suitable deposition technique. If theinsulating layer is an oxide, the insulating layer may undergo anoxidation treatment subsequent to deposition. The second magnetic layerthen is provided on the insulating layer by a further depositiontechnique. Patterning techniques such as masking, etching and/orphotolithography may be used to create the desired geometrical shape.

External magnetic fields may be applied to the MTJ disclosed herein byemploying wiring configurations. Typically, a plurality of memory cellseach using the above-described MTJ are arranged in matrix form and upperand lower wiring layers are laid over and under the memory cells. Theupper and lower wiring layers are substantially perpendicular with eachother while being spaced apart at a predetermined distance (e.g., thethickness of the memory cells) from each other. One of the wiring layersis formed of a low-electrical-resistance conductive material that ispatterned to form a plurality of bit lines in a predeterminedconfiguration. Similarly, the other wiring layer is formed of alow-electrical-resistance conductive material that is patterned to forma plurality of word lines in a predetermined configuration, whichintersect the bit lines at right angles.

As mentioned above, each memory cell in the MRAM has two magneticlayers. One is a free or storage layer having a direction ofmagnetization changed according to the direction of an external magneticfield. The free layer may be electrically connected to the correspondingbit line or word line. The other layer is a pinned layer having a fixeddirection of magnetization. The pinned layer may be electricallyconnected to the corresponding word line or bit line. When informationis recorded (written) on a selected memory cell, the word and bit lineselectrically connected to the memory cell are selected and predeterminedwrite currents are caused to flow respectively through the word and bitlines. These write currents induce magnetic fields around the word andbit lines according to the values of the write currents. The directionof magnetization of the free layer changes according to a resultantmagnetic field formed by the two induced magnetic fields.

If the direction of magnetization of the free layer changed in thismanner is the same as the direction of magnetization of the pinned layerof the same memory cell, the directions of the free layer and the pinnedlayer are in parallel. If the changed direction of magnetization of thefree layer is opposite to the direction of magnetization of the pinnedlayer, the directions of the free layer and the pinned layer are inanti-parallel.

The direction of magnetization of the free layer is thus changed towrite binary information “0” or “1” in the selected cell. To change thevalue written in the selected memory cell, one of the write currentscaused to flow through the word and bit lines is reversed in direction(inverted). The direction of resultant magnetic field induced around theword and bit lines by the two write currents is thereby changed toreverse the direction of magnetization of the free layer, i.e., to writethe other value.

An example of a wiring configuration is depicted in FIG. 12. An array ofMTJ elements 20 are arranged at the intersections of a plurality of wordlines 21 and bit lines 22. The MTJ elements 20 are arranged so that theopen end of each gap or notch section is pointing in substantially thesame direction, thus providing uniform single domain magnetizationacross the array configuration. The MTJ element 20 is configured so thatthe free layer is in the same plane as the word line 21 and the pinnedlayer is in the same plane as the bit line 22. An electricallyinsulating layer 23 is provided adjacent to, and perpendicular to, thebit lines 22. A plurality of digit lines 24 are provided adjacent to theelectrically insulating layer 23, and perpendicular to the bit lines 22.A current is supplied to the digit lines 24 in conjunction with acurrent supplied to the bit lines 22 for inducing a first magneticfield. This induced first magnetic field can switch (write) a MTJelement 20.

A further example of a wiring configuration is illustrated in FIGS. 13Aand 13B in which electrical connections of MTJ elements 30 withtransistors 36 are shown. An array of MTJ elements 30 are arranged atthe intersection of bit lines 34 and word lines 35. The MTJ elements 30are arranged so that the open end of each gap or notch section ispointing in substantially the same direction, thus providing uniformsingle domain magnetization across the array configuration. A digit line31 is provided perpendicular to the bit line 34. An electrode 32 isdisposed around the outer periphery of the MTJ element 30. The electrode32 serves as a contact point for line(s) leading to the transistor 36. Alow k dielectric-insulating layer 33 is disposed between the electrode32 and the digit line 31. The digit lines 31 and the bit lines 34 canproduce a magnetic field to switch the MTJ elements 30 as describedabove. More specifically, current flowing through the digit line 31produces a magnetic field along the easy direction (i.e., perpendicularto the direction of the open end of the gap or notch in the MTJ element30). Simultaneously, current flowing through the bit line 34 produces amagnetic field along the hard direction that is perpendicular to themagnetic field produced by the digit line 31. Reading of the MTJ element30 may be performed by a combination of the bit line 34 and the wordline 35. More specifically, the current flowing through the word line 35determines the status of the current flowing through the bit line 34 andthe MTJ element 30, by turning on or off the transistors 36. The on-offbit signal is determined by comparing the electrical resistance of theaddressed MTJ element 30 to that of a reference cell (not shown).

The unique shapes of the magnetic elements described herein can providean improved low switching field and an exceptionally narrow switchingfield distribution. For example, FIG. 14 (described in more detailbelow) shows a switching field distribution of approximately 90 Oe. Theswitching field distribution of the element is dependent upon thegeometry of the magnetic element, especially the gap distance d, theinner radius r, the outer radius R, the angle α, and the layer thicknesst. In general, the switching field distribution increases withincreasing layer thickness t, increasing inner radius r, increasing gapdistance d, decreasing the outer radius R, and decreasing the angle α.Thus, a desired switching field range can be easily achieved byappropriately designing the geometry of the magnetic element.

The elements may be characterized by an asymmetric circularmagnetization mode as depicted by the arrows in FIGS. 1 and 2, or amodified linear mode depending upon the geometry of the notch or gapsection. Moreover, the layers may exhibit single domain magnetization,are substantially free of edge magnetic domain, 360° domain walls, andlocalized vortices. Although not bound by any theory, it is believedthat these properties flow, at least in part, from the shape anisotropyof the layers. The magnetization states of the magnetic layers of theMTJ can be controlled by in-plane external magnetic fields andsubstantially no vortex core exists. The magnetic elements describedherein also can increase the information data storage density of MRAMdevices.

The specific examples described below are for illustrative purposes andshould not be considered as limiting the scope of this disclosure.

EXAMPLES

The magnetic domain configuration of several notch- or gap-shapedmagnetic elements were observed under a magnetic force microscope (MFM)and an atomic force microscope (AFM). The domain configurations atremanent state were studied by MFM for the switching behavior of themagnetic elements under a successively applied magnetic field.

The magnetic elements were made by initially forming a positivepoly(methyl methacrylate) (PMMA) mask pattern on a naturally-oxidized Siwafer. The PMMA mask was fabricated using electron beam processing. Ananometer film was then deposited on the geometrical area not covered bythe PMMA via conventional rf non-magnetron sputtering. The filmcomprised an adhesive layer of 35 nm of Ti adjacent to the Si surface, amiddle layer of 32 nm of Co, and a cap layer of 35 nm of Ti disposed ontop of the Co layer. No magnetic field was applied during thedeposition. The PMMA then was removed by etching.

The MFM images of FIG. 10 demonstrate that the uniquely-shaped magneticelements exhibit a uniform single magnetic domain since no boundaries(i.e., walls) between domains are visible. FIG. 10A is an AFM image of amagnetic element configured with a slot-shaped gap. FIG. 10B is an AFMimage of a magnetic element configured with a gap section that defines aslot-shaped portion and a circular-shaped portion. FIGS. 10C and 10D areMFM images of the magnetic elements shown in FIGS. 10A and 10B,respectively.

The MFM images of FIG. 11 illustrate the magnetic domain configurationsat remanent state after successively applied magnetic fields. A field of−400 Oe was initially applied in the −y direction to saturate themagnetic element, and then a successive field was applied in the +ydirection. FIG. 11A is an AFM image of a magnetic element configuredwith a slot-shaped gap. FIG. 11B is an AFM image of a magnetic elementconfigured with a gap section that defines a slot-shaped portion and acircular-shaped portion. FIGS. 11C and 11D are MFM images of themagnetic elements shown in FIGS. 11A and 11B, respectively, afterapplication of an initial −400 Oe field. FIGS. 11E and 11F are MFMimages of the magnetic elements shown in FIGS. 11A and 11B,respectively, after application of an initial −400 Oe field followed bya +252 Oe field. FIGS. 11G and 11H are MFM images of the magneticelements shown in FIGS. 11A and 11B, respectively, after application ofan initial −400 Oe field followed by a +271 Oe field. The magneticdomain configuration results indicate that the magnetic element having agap section that defines a slot-shaped portion and a circular-shapedportion has a lower switching field compared to the magnetic elementhaving only a slot-shaped gap.

The switching field distribution of magnetic elements having awedge-shaped gap section was observed by scanning the magnetic domainconfigurations utilizing a MFM. The number of switched elements in anarray of 100 elements were counted after successively applied magneticfields. The successively applied fields were +440 Oe, remanent state (0Oe), a selected negative field, and then back to a remanent state (0Oe). The resulting remanence curve is shown in FIG. 14.

1. A magnetic tunneling junction comprising: a first magnetic layer anda second magnetic layer, each magnetic layer defining an outerperipheral profile and a center point, wherein the outer peripheralprofile includes a substantially curviform section and a notch sectionconfigured to radially extend to at least the center point; and aninsulating layer interposed between the first magnetic layer and thesecond magnetic layer.
 2. The magnetic tunneling junction of claim 1,where the notch section defines a wedge-shaped profile.
 3. The magnetictunneling junction of claim 1, wherein the notch section defines aslot-shaped profile that includes an arcuate closed end.
 4. The magnetictunneling junction of claim 1, wherein the notch section defines aparabolic-shaped profile.
 5. The magnetic tunneling junction of claim 1,wherein the notch section defines a slot-shaped portion and acircular-shaped or oval-shaped portion.
 6. The magnetic tunnelingjunction of claim 1, wherein the first magnetic layer and the secondmagnetic layer have the same outer peripheral profile shape.
 7. Themagnetic tunneling junction of claim 6, wherein the insulating layerdefines an outer peripheral profile having the same shape as the outerperipheral profile of the first magnetic layer and the second magneticlayer.
 8. The magnetic tunneling junction of claim 1, wherein the outerperipheral profile further includes a straight section opposing thenotch section.
 9. A magnetic random access memory device comprising themagnetic tunneling junction of claim
 1. 10-16. (canceled)
 17. A magneticrandom access memory device comprising the magnetic tunneling junctionof claim
 1. 18. A magnetic element defining an outer peripheral profileand a center point, wherein the outer peripheral profile includes asubstantially curviform section and a notch section configured toradially extend to at least the center point.
 19. (canceled)